Clarification on the various types of dB

[jmaudio]

Hi,

 

I've been doing sound now for about 3 years now, and recently I have been getting some work with a nearby rental company - I'm learning a lot!

 

However, there are a couple of things that are probably easier to ask on a forum, instead while setting up/striking a show! This I think is one of them.

 

Basically I just want to understand how the various types of dB scale work, and how they are all tied together. I've had a look on the wiki, and searched the internet, but couldn't find much information.

 

Also, if someone could explain the various reference points of the dB scales, for example, dBSPL - what is 0dBSPL etc, that would be brilliant.

 

And all the other ones that are commonly used in this industry: dBv dBu dBFS. Any others? :)

 

Thanks in advance,

 

Jamie.

[p.k.roberts]

You might find this article useful ...........

 

http://www.soundonsound.com/sos/1994_artic...4/decibels.html

[mervaka]

dB is a logarithmic relative measure.

 

with no suffix, it's simply a measure of gain relative to the input. 0dB means unity gain. positive values represent amplification, negative values represent attenuation. in audio terms, if something is 6dB larger then it is twice the size. if something is 20dB larger then it is 10 times the size, and so on.

 

the various suffixes are different for different applications. dBA, dBC etc are acoustic measures of SPL relative to the threshold of human hearing. Read Here.

 

dBV and dBu are electrical measures relative to 1V and 0.775V. You need not worry about this until you delve into the electronics of audio, as two standards exist for analogue audio: +4dBV and -10dBu. just remember +4dBV has the hotter signal!

 

dBFS is a measure of attenuation relative to the biggest possible value you can store. anything above 0dBFS is just clipped. for example, a 16 bit audio sample has a dynamic range of about 96dB, while a 24 bit audio sample has about 144dB. these are the smallest signals a 16bit or 24bit sample can represent.

[boatman]

You might find this article useful ...........

 

http://www.soundonsound.com/sos/1994_artic...4/decibels.html

 

Just a pity that there us a fundamental error in that article:

 

Number of dBs = 20 log (P1/P2) where P1 and P2 are the two powers being compared and where the log is to the base 10.

 

should read:

 

Number of dBs = 20 log (V1/V2) where V1 and V2 are the two voltages being compared and where the log is to the base 10.

 

The first equation regarding powers is correct:

 

Number of dBs = 10 log (P1/P2) where P1 and P2 are the two powers being compared and where the log is to the base 10.

[Simon Lewis]

The definition of a decibel is 10 log (power 1 / power 2).

This is just a dimensionless ratio, and this holds true for acoustic power or indeed electrical power.

 

For sound power level, the reference of 0dB is 1x10^-12W

 

SWL = 10log(W/1x10^-12)

 

For sound intensity level the reference of 0dB is 1x10^-12W/m²

 

SIL = 10log(I/1x10^-12)

 

For sound pressure level, the reference of 0dB is 2x10^-5 N/m²

 

If we look at the plane wave equation, we see that sound power is proportional to sound pressure squared.

 

Therefore, we could write SPL = 10log (P²/Pref²), but it's easier to use:

 

SPL = 20log(P/Pref) or 20 log (P/2x10^-5)

 

Similarly, electrical power is proportional to the voltage squared, so

 

voltage gain = 20 log (V1 / V2) (dimensionless) or using a reference level

 

0dBu = 0.775V

0dBV = 1.0V

 

Hence:

dBu = 20log (V/0.775)

dBV = 20log (V/1.0)

 

There will be a test later :D

[AndyL]

Similarly, electrical power is proportional to the voltage squared, so

 

voltage gain = 20 log (V1 / V2) (dimensionless) or using a reference level

 

0dBu = 0.775V

0dBV = 1.0V

 

Hence:

dBu = 20log (V/0.775)

dBV = 20log (V/1.0)

 

There will be a test later :D

 

Why 0.775V you ask ? (OK you didn't ask but here's why anyway)

 

dBm = dB relative to 1mW

 

But in order to know what Voltage that represents we need to know the impedance that is disipating the 1mW

 

Once upon a time everything in Pro Audio, and before that in telephony, revolved around an impedance of 600ohms.

 

1mW in 600ohms = 775mV.

 

n dBu indicates that we are expecting the same voltage as we would get at n dBm in a 600 ohm system, regardless of the impedances involved.

0dBu = 0.775V = 1mW in 600 ohms = 0dBm

 

In the world of RF 0dBm still means 1mW but the reference impedance will usually be 50 ohms.

 

0dBW is dB relative to 1W.

0dBW = 1W = +30dBm

 

dBFS stands for dB relative to "Full Scale", i.e. 0dBFS means that all the bits in your digital system are being fully occupied and you have zero digital headroom. Each bit in a digital audio system represents 6dB. It is important when converting from Analogue to Digital or vice versa to know what 0dBFS represents in the analogue world on the equipment that you are using. 0dBFS can convert to an awful lot of dBu, or Volts, at the output of some digital desks!

[Bobbsy]

dB or not dB...that is the question!

 

Seriously, time for my usual plug for the Yamaha Sound Reinforcement Handbook which contains most of what you need to know about this topic, including lots of the necessary formulae.

 

Bob

[djandydee]

With UK mains sitting at +49.448 dBu

[RoyS]

Jamie,

 

Go and have a talk with Nev at TRS who has the Yamaha book mentioned above.

 

I'm very sure he'd lend it to you...